This invention relates to a new class of manganese oxide octahedral molecular sieves (OMS) possessing a (3.times.4) tunnel structure and to a method for their production.
Manganese oxide octahedral molecular sieves possessing mono-directional tunnel structures constitute a family of molecular sieves wherein chains of MnO.sub.6 octahedra share edges to form tunnel structures of varying sizes. Such materials have been detected in samples of terrestrial origin and are also found in manganese nodules recovered from the ocean floor. Manganese nodules have been described as useful catalysts in the oxidation of carbon monoxide, methane and butane (U.S. Pat. No. 3,214,236), the reduction of nitric oxide with ammonia (Atmospheric Environment, Vol. 6, p. 309 (1972)) and the demetallation of topped crude in the presence of hydrogen (Ind. Eng. Chem. Proc. Dev., Vol. 13, p.315 (1974)).
The hollandites are naturally occurring hydrous manganese oxides with tunnel structures (also described as "framework hydrates") in which Mn can be present as Mn.sup.4+ and other oxidation states, the tunnels can vary in size and configuration and various mono- or divalent cations can be present in the tunnels. The hollandite structure consists of double chains of MnO.sub.6 octahedra which share edges to form (2.times.2) tunnel structures. The average size of these tunnels is about 4.6 .ANG. square. Ba, K, Na and Pb ions are present in the tunnels and coordinated to the oxygens of the double chains. The identity of the tunnel cations determines the mineral species. Specific hollandite species include hollandite (BaMn.sub.8 O.sub.16), cryptomelane (KMn.sub.8 O.sub.16), manjiroite (NaMn.sub.8 O.sub.16) and coronadite (PbMn.sub.8 O.sub.16).
The hydrothermal method of synthesizing a manganese oxide octahedral molecular sieve possessing (2.times.2) tunnel structures such as those possessed by the naturally-occurring hollandites is described in "Hydrothermal Synthesis of Manganese Oxides with Tunnel Structures," in Synthesis of Microporous Materials, Vol. II, 333, M. L. Occelli, H. E. Robson Eds. Van Nostrand Reinhold, N.Y., 1992. Such synthetic octahedral molecular sieves having (2.times.2) tunnel structures are referred to in the art by the designation OMS-2. The (2.times.2) tunnel structure of OMS-2 is diagrammatically depicted in FIG. 1A.
The hydrothermal method of producting OMS-2 involves autoclaving an aqueous solution of manganese cation and permanganate anion under acidic conditions, i.e., pH&lt;3, at temperatures ranging from about 80.degree. to about 140.degree. C. in the presence of counter cations having ionic diameters of between about 2.3 and about 4.6 .ANG.. The counter cations can serve as templates for the formation of OMS-2 product and be retained in the tunnel structures thereof. Based on analytical tests, OMS-2 produced via this method is thermally stable up to about 600.degree. C.
Alternatively, OMS-2 can be produced by the method disclosed in R-Giovanili and B. Balmer, Chimia, 35 (1981) 53. Thus, when manganese cation and permanganate anion are reacted under basic conditions, i.e., pH&gt;12, a layered manganese oxide precursor is produced. This precursor is ion exchanged and then calcined at high temperatures, i.e., temperatures generally exceeding about 600.degree. C., to form OMS-2 product. Analytical tests indicate that OMS-2 produced via this method is thermally stable up to about 800.degree. C. and the average oxidation state of manganese ion is lower.
The todorokites are naturally occurring manganese oxides with (3.times.3) tunnel structures formed by triple chains of MnO.sub.6 edge-sharing octahedra. Todorokites and related species are described by Turner et al. in "Todorokites: A New Family of Naturally Occurring Manganese Oxides", Science, Vol. 212, pp. 1024-1026 (1981). The authors speculate that since todorokites are often found in deep-sea manganese nodules containing high concentrations of copper and nickel, it is probable that such metals substitute for Mn.sup.+2 in the octahedral framework.
Todorokites have attracted particular interest because of their relatively large tunnel dimension and their cation-exchange behavior which is similar to that of zeolites (Shen et al., "Manganese Oxide Octahedral Molecular Sieves: Preparation, Characterization, and Applications", Science, Vol. 260, pp. 511-515 (1993)). The naturally occurring todorokites are poorly crystalline, impure in composition and coexist with other manganese oxide minerals. Results of high resolution transmission electron microscopy (HRTEM) show that todorokite contains random intergrowth material of 3.times.2, 3.times.3, 3.times.4 and 3.times.5 tunnel structure. Because of their disordered structure, the todorokites exhibit variable and non-reproducible catalytic activity, a drawback which militates against their commercial use.
A method of synthesizing a manganese oxide octahedral molecular sieve possessing (3.times.3) tunnel structures such as those possessed by the naturally-occurring todorkites is described in U.S. Pat. No. 5,340,562. Such synthetic octahedral molecular sieves having (3.times.3) tunnel structures are referred to in the art by the designation OMS-1. The (3.times.3) tunnel structure of OMS-1 is diagrammatically depicted in FIG. 1B.
OMS-1 can be prepared by reacting manganese cation and permanganate anion under strongly basic conditions to form a layered manganese oxide precursor, thereafter aging the precursor at room temperature for at least 8 hours, ion-exchanging the aged precursor and then autoclaving the ion-exchanged precursor at from about 150.degree. to about 180.degree. C. for several days. Analytical tests indicate that OMS-1 produced via this method is thermally stable up to about 500.degree. C.
Methods of substituting the frameworks of OMS-1 and OMS-2 with a metal other than manganese are described in commonly assigned, copending U.S. appln. Ser. No. 08/215,496.